Summary: There is an urgent need to develop effective treatments for autism, a brain disorder that affects 1 in 68 children. The development of targeted therapies is hindered by the large diversity of underlying causes for autism. Yet, recent research shows that certain molecular pathways, e.g. the phosphoinositide 3-kinase (PI3K) pathway, are more frequently affected, raising hope that they could be disease-modifying targets with broader applicability. Prominent examples of PI3K regulators implicated in autism include Fragile X Mental Retardation Protein, which translationally suppresses PI3K pathway components, and phosphatase and tensin homolog deleted on chromosome ten (PTEN), which counteracts PI3K activity by dephosphorylating phosphoinositide triphosphate (PIP3). PTEN is an autism risk gene but also often mutated in cancer, where research into molecular mechanisms is already more advanced. This provides an opportunity to utilize treatment strategies developed in cancer research for autism. By taking advantage of the progress in PTEN cancer biology and work in mouse models of Fragile X Syndrome (FXS) the proposed project will test a novel disease mechanism-based therapeutic strategy for PTEN-associated neurodevelopmental disorders. To date, pre-clinical studies of PTEN- associated brain disorders have mainly targeted the signaling hub mTOR, which is only one out of several PTEN downstream targets. Inhibiting PI3K, the direct opponent of PTEN's lipid phosphatase activity might be a more effective strategy because it will correct an immediate consequence of PTEN deficiency, the accumulation of PIP3. PTEN-opposing PI3K activity is mediated by four different PI3K catalytic isoforms (p110?,?,?,?), but several studies have shown that PTEN-deficient tumors mostly depend on the isoform p110?, whereas p110? and ? are less important and p110? may not have a role at all. Notably, the applicants' published and unpublished data show that p110? reduction rescues a broad spectrum of phenotypes in FXS mice, suggesting an important role of p110? not only in PTEN-deficient cancers but also in neuronal function. P110?-selective inhibitors are currently evaluated in clinical trials for PTEN-deficient cancers, but the role of p110? or any of the other isoforms in PTEN- associated neuronal defects, including autism and epilepsy is unknown. The overall hypothesis of this application is that inhibiting select PI3K isoforms may be therapeutic in PTEN-associated brain disorders, and that p110? is a major mediator of neuronal defects associated with PTEN deletion and thus a particularly promising target. Combining the investigators' strengths in studying molecular defects underlying altered PI3K activity in FXS and their expertise in PTEN-associated neuronal phenotypes, this hypothesis will be tested in three aims. Pharmacological and genetic tools in neuron-specific Pten deletion mice will be used to assess which p110 isoform drives increased PI3K signaling in neurons (aim 1), if elevated PIP3 plays a role in altered neuronal morphology (aim 2), and if a p110?-selective inhibitor rescues autistic-like behavior and seizures (aim 3). Positive results will suggest repurposing p110? inhibitors, developed to treat cancer, for neurodevelopmental disorders.